1. Field of Invention
This invention relates to device and illumination independent color reproduction.
2. Description of Related Art
Generally, colors are defined in two ways, in device dependent color spaces and in device independent color spaces. To illustrate, most color display monitors, such as, for example, color computer monitors, display colors in the red/green/blue (RGB) color space, i.e., with respect to the amount of red, green, and blue that a particular displayed color contains. Using this technique, the color yellow, for example, is displayed on a color display monitor by combining a red image value of 100 percent red with a green image value of 100 percent green and a blue image value of zero percent.
Furthermore, the red, green, and blue (RGB) color values associated with the particular colors for a color display monitor are device dependent. This means that the RGB values associated with a particular color, viewed on a specific color display monitor, are unique to that specific color display monitor or, at least, to that brand of color display monitor. Simply put, because RGB color values are device dependent, if identical RGB color values, such as, for example, a red image value of 100 percent red, a green image value of 100 percent green, and a blue image value of zero percent, are input and displayed on two different color display monitors, the resulting yellow color displayed on the two color display monitors will probably not appear exactly alike.
Similarly, most color marking devices, such as, for example, color printers, print colors in device dependent terms. However, unlike most color display monitors, most color marking devices use a cyan, magenta, yellow, and black (CMYK) color space, i.e., a combination of cyan (C), magenta (M), yellow (Y) and black (K) (CMYK) to arrive at the color marking device's printed colors. Consequently, as with RGB color values, CMYK color values are device dependent. Thus, as described above with respect to colors being displayed on color display monitors, if identical CMYK colors are printed by two different color marking devices; the printed colors will probably not appear exactly alike.
The other way of describing color is in device independent color spaces. By describing color in a device independent color space, consistent colors can be reproduced regardless of the type of device that is used to display or print the color. Therefore, color reproduction is generally done by defining colors in a device independent parameter space, such as, for example, L* a* b*, X Y Z, or L h v.
However, using device independent parameters for color reproduction does not eliminate the color matching problem. Device independent parameters remain illumination dependent and observer dependent. To illustrate, different illuminants, or lighting conditions, such as, for example, florescent lights, incandescent lights, or ordinary daylight, have their own spectral characteristics. Therefore, when a color document is viewed under different lighting conditions, the colors of that particular color document can show variations in color tone, saturation, and hue. The level of variation can be slight and barely noticeable, or the level of variation can be extreme and very noticeable. This is because a color's appearance is affected by the spectral characteristic of the particular light source that is illuminating the color.
Therefore, precisely reproducing a color using a device independent trichometric space parameter, such as, for example, L* a* b*, X Y Z, or L h v requires specification of both an illuminant, such as, for example, fluorescent light, and an observer, such as, for example, the user. Unfortunately, this technique only achieves a colorimetric match for the specified observer with the specified illuminant.
Thus, colorimetrically matched prints, such as, for example, prints matched to a device independent L* a* b* space from two different color printers exhibit varying amounts of similarity to each other under different lighting conditions. To illustrate, if a test image is printed using two similar printers that are calorimetrically matched using known device independent color control techniques, when the printed test images are compared to each other, side by side, under various lighting conditions, the color of the test image from the first printer will appear different from the color of the test image from the second printer. The colors will appear different unless the prints are calorimetrically matched, provided the observer and illumination spectra are matched between the colorimetric quantities of the two printers. The amount of noticeable difference between the two test prints varies, in part, depending on the colors that the test prints contain. These variations are especially noticeable when the test image contains an abundance of the color red. In color science, this phenomenon is well known and is called metamerism.
To add to the complexity of color matching between color marking devices, different color marking devices can use different types of toners, dyes, pigments, or inks to produce the outputted color images. Likewise, the color images can be produced on a wide range of copy media. Images can be produced, for example, on copy media ranging from paper to plastic or from fabric to metal. In each case, each combination of colorant and media produces a different optical appearance.
Moreover, different color devices have different color capabilities. Every color device, whether it is a color scanner, a color marking device, or a color display monitor, has a color gamut, i.e., a range of colors that it can capture, produce, or display. To illustrate the problems encountered when color matching is attempted between two different devices having two different color gamuts, consider color display monitors and color marking devices. Most color display monitors can display hundreds of thousands of colors. Conversely, color marking devices usually have a significantly smaller number of producible colors. Therefore, the gamut of a color display monitor usually exceeds the gamut of a color marking device. Thus, some of the colors that can be displayed on a color display monitor cannot be printed by a color marking device.
In an attempt to solve the problem of color matching, various color matching techniques have been developed that use models to translate colors from one color space to another color space. These models usually manifest themselves in the form of predetermined multi-dimensional look-up tables. These predetermined multi-dimensional look-up tables translate colors from one color space to another color space while attempting to maintain the translated color's perceived appearance. For example, if a user creates an image on a color display monitor and subsequently prints the created image without any color matching, the colors observed on the printed image may differ significantly from the colors originally observed on the color display monitor. However, if some type of color matching model is used, the discrepancies between the colors originally observed on the color display monitor and the colors observed on the printed image can be reduced.
One method of creating and updating a multi-dimensional look-up table is described in U.S. Pat. No. 6,157,469, incorporated herein by reference in its entirety. The incorporated 469 patent discloses a method of reducing and controlling color drift between a desired image, and an output image printed by a marking device that is intended to match the desired image, by detecting a current output color in the output image with a color sensing device. A difference between the current output color in the output image and a corresponding color in the desired image is then determined. A next output color in the output image is then automatically set equal to a corrected color that minimizes the difference between the next output color and the corresponding color in the image. This is preferably done on a real-time basis.
Additionally, in U.S. Pat. No. 6,236,474, incorporated herein by reference in its entirety, the error in an output color of a colored output image in a marking device intended to match a desired image is reduced. The method includes detecting a current output color in the output image with a color sensing device. A difference between the current output color and a corresponding target color under standard conditions is then determined. A marking device input-output relationship for a next output color is then automatically set based on the difference between the current output color and the corresponding target color under standard conditions to minimize the difference between the next output color and the corresponding target color.
Furthermore, in U.S. Pat. No. 6,052,195, incorporated herein by reference in its entirety, colorants are mixed to achieve a target color by combining individual colorants, detecting an output color of the combined colorants with a color sensing device and automatically adjusting the output color based on comparison between the detected output color and the target color.